The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
During a borehole drilling operation there is a need to survey the path of the borehole to determine if its course is being maintained within acceptable limits. Surveying a borehole is usually accomplished using a surveying tool which is moved along the borehole to obtain the information required, or at least data from which the required information can be determined. Information relating to the path of a borehole can typically include inclination, azimuth and depth.
Surveying tools typically contain sensor devices for measuring the direction and magnitude of the local gravitational field and also the direction and magnitude of the rate of rotation of the Earth. These measurements correspond to the orientation of the surveying tool in the borehole. The position, inclination and/or azimuth can be calculated from these measurements.
The sensor devices can comprise accelerometers for measuring the direction and magnitude of the local gravitational field, and gyroscopes for measuring direction and magnitude of the rate of rotation of the Earth, from which azimuth can be calculated.
Commercially available gyroscopes contain systematic errors which can seriously affect the accuracy of measurement. Such errors can be removed by indexing the gyroscope.
In order to index sensor devices between various indexing positions, there is a need for an indexing mechanism aboard the surveying tool.
The need to index and orient the sensor devices can introduce cost and complexity to the surveying tool, and can be particularly problematic where a survey tool/instrument of compact construction is required. Furthermore, known indexing arrangements usually require direct mechanical drive using a stepper motor or servo motor with precision position encoding resulting in additional cost and technical complexity.
In some instances, the use of conventional motor drives and related mechanical configurations used for indexing purposes can reduce the precision of information recorded during a survey of a borehole. In this regard, induced vibrational and/or shock forces occurring during measurement and/or indexing can compromise the data recorded by sensors included with the surveying tool (especially if using a servo control loop for positioning the sensors).
It is against this background that the present invention has been developed.